In addition to the points other commenters made about 2160p video usually being distributed at a higher bitrate, most (if not all) digital video you’ve ever seen (web video, DVDs, Blu-rays, UHD Blu-rays, …) uses a process called [YCbCr 4:2:0 chroma subsampling](https://en.wikipedia.org/wiki/Chroma_subsampling). The idea is that because the human eye is more sensitive to changes in brightness than changes in color tone, only the brightness portion of the image is stored at full resolution. Each 2×2 block of brightness samples shares a color tone, so while the brightness information is 2160p, the color information is only 1080p. If you’re watching 1080p video, its color information is stored at 540p.
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